Applying the flame retardant LDH as a Trojan horse for molecular flame retardants

• Direct synthesis of Mg2AlCl-LDH with an NH4Cl buffer to control pH.
• pH during LDH synthesis was shown to affect CO32 − concentration in solution.
• Mg2AlCl-LDH undergoes a topotactic anion exchange with phenyl phosphate (PP).
• The obtained LDH-PP shows hydration with 2 distinct hydrated phases.
• LDH-PP also enhances the flame retardant effect of the LDH on polystyrene.

The direct synthesis of Mg2Al–Cl LDH (Cl-LDH) has been accomplished by combining the urea method with the addition of NH4Cl as a buffer. It has been shown, that the buffer limits the maximum pH reached during urea hydrolysis, which in turn limits the concentration of carbonate in the reaction mixture and allows for recovery of Cl-LDH. By anion exchange into Cl-LDH phenyl phosphate (PP) was incorporated as a model flame retardant. The mechanism of this anion exchange could be shown to be topotactic rather than of dissolution/re-precipitation type. Despite hydration of PP-LDH up to a d-value of 15.8 Å, no exfoliation could be observed upon applying high shear forces. With a filler content of 5 wt.%, PP-LDH could be shown to significantly push the flame retardant properties of polystyrene as compared to ordinary CO3-LDH as indicated by a doubling of the reduction of the peak of heat release rate (22% and 47%, respectively).